Method of improving the burn rate and ignitability of aluminium fuel particles and aluminum fuel so modified

ABSTRACT

A method of improving the burn rate and ignitability of aluminium fuel particles, and a thus modified aluminium fuel for use in propellant and explosive compositions and pyrotechnic charges. Aluminium fuel particles are treated with an aqueous solution of hydrofluoric acid and a fluoride and/or complex fluoride salt of an alkali metal and/or alkaline earth metal to form a surface layer of a fluoride complex bound to the aluminium fuel particle.

This is a nationalization of PCT/SE03/001842 filed Nov. 28, 2003 andpublished in English.

The invention relates to a method of improving the burn rate andignitability of aluminium fuel particles and also a thus modifiedaluminium fuel for use in propellant and explosive compositions andpyrotechnic charges.

Particulate aluminium fuel, for instance in the form of powder, flakesor fibres, is used, inter alia, in rocket propellant to achieve a highspecific impulse and in explosive compositions to increase the capacityof the charge. A drawback of aluminium fuel is that it requires a veryhigh ignition temperature. A first condition for combustion is that theoxidising agent, i.e. oxidising gases, can come into contact with thefuel. Aluminium has naturally a protective oxide layer on its surface.It is this oxide layer that allows aluminium to mix with oxidisers andexplosives in propellant and explosive compositions without any greatrisks since the element in itself is highly reactive. However, the oxidelayer is a great obstacle in combustion since it prevents the oxidisingagent from coming into contact with the fuel. The surface of thealuminium particle must usually be heated until the oxide layerevaporates, which requires a temperature above 2000° C. Such a hightemperature must also be maintained during the entire combustion processsince otherwise a new oxide layer forming extinguishes the aluminiumparticle. The high temperature must initially be provided by combustionof other energetic materials in the composition, which restricts thepossible choice of these materials. Also a small reduction of theignition temperature makes a great difference in the ignitability of thealuminium fuel. If the limit temperature can be lowered by just somehundred degrees, it means that a much larger number of energeticmaterials can effectively ignite the aluminium fuel.

It is previously known to mix fluorides, above all alkali metalfluorides, in propellant compositions containing metal fuel in order toimprove the combustion properties of the composition. However, quitelarge additions have been required for effect. Since metal fluorides inthemselves are not an energetic material, large additions of fluorideresult in a reduction of the performance of the composition.

U.S. Pat. No. 4,017,342 discloses a method of improving the combustionproperties of aluminium powder by treating the powder with hydrogenfluoride gas. The gas molecules are said to diffuse through the oxidelayer and react with the pure aluminium underneath and, after treatmentfor some time, result in an aluminium fluoride coating, which lowers theignition temperature of the metal powder and increases its burn rate.However, the method suffers from process-technical drawbacks since it isa gas-solid phase reaction that involves the handling of a highlyaggressive hydrogen fluoride gas.

The object of the present invention is to provide additional improvementof ignitability and burn rate of a particulate aluminium fuel. Anotherobject is to provide an improved aluminium fuel by a simple treatment ofaluminium fuel particles with an aqueous solution.

This is achieved by a method and an aluminium fuel as defined in theclaims.

According to the invention, the particulate aluminium fuel is treatedwith an aqueous solution of hydrofluoric acid and a fluoride and/orcomplex fluoride salt of an alkali metal and/or an alkaline earth metal.The solution reacts with the oxide layer of the particle and causes asurface layer of a fluoride complex on the fuel particle.

When igniting the particle, the fluoride complex melts and dissolves anyrests of the oxide layer and then evaporates at a relatively lowtemperature so that pure aluminium is exposed to reaction with oxidisinggases.

The process is simple to perform. Aluminium fuel particles, such as apowder, are brought into contact with the treatment solution, the powderbeing etched by the diluted hydrofluoric acid which can dissolve thealuminium oxide on the surface of the particles and replace oxygen atomswith fluorine. The aluminium is then dissolved as aluminium trifluoridein the acid but since at the same time the solution contains ionsreacting with the produced aluminium fluoride to fluoride complex, aprotective layer of this fluoride complex forms on the surface of thealuminium particles.

Preferably an aqueous solution is used, having the molar ratio 1:1 withregard to alkali metal/alkaline earth metal salt and hydrofluoric acid.The concentration of the solution with regard to fluoride may varywithin wide limits, for instance 0.01-10 moles per litre. The aluminiumparticles can be added to the treatment solution, or the treatmentsolution can be added to a suspension of the aluminium particles inwater. In the latter case, a more concentrated treatment solution issuitably used. The temperature can be 0-100° C. depending on thecomposition of the treatment solution. Some fluoride complexes, such astrisodium hexafluoroaluminate, are partly soluble in water and thesolubility increases with increasing temperature, and therefore a lowertreatment temperature, for instance 25-40° C., should be used in somecases. As will be illustrated below, however, the treatment solution canbe modified with regard to the solubility of the fluoride complex sothat also surface layers of partly soluble fluoride complexes can beproduced at a higher temperature. The reaction temperature affects thethickness of the surface layer, and the surface layer will usually bethicker the higher the treatment temperature. A higher concentration ofthe solution also results in a thicker layer.

In the treatment, hydrogen gas develops for a short period as thereaction starts. The coating forms very quickly and the process stops byitself when a protective layer of fluoride complex has formed on thesurface of the particles. The fuel particles can then be filtered offfrom the solution and dried. In the process, it must be ensured that thesolution remains acidic in order to prevent a basic reaction betweenhydroxide ions and aluminium.

Suitable alkali metal fluorides in the treatment solutions are sodium,potassium, rubidium or cesium fluoride. Sodium or potassium isparticularly preferred. When the counter-ion is sodium, a surface layerof cryolite forms, which is a well-known solvent for aluminium oxide.When the counter-ion is potassium, the process is still easier toperform since the formed surface coating, tripotassiumhexafluoroaluminate, has less solubility in the acid solution thancryolite. The surface layer will be denser and protects the fuelparticle in a better way. Tripotassium hexafluoroaluminate melts at alower temperature than cryolite and causes a greater reduction of theignition temperature of the aluminium fuel.

The complex fluoride in the treatment solution preferably is ahexafluoroaluminate, AlF₆ ³⁻, or a hexafluorosilicate, SiF₆ ²⁻. When thetreatment solution contains, in addition to HF, merely an alkali metalfluoride, a relatively large amount of aluminium must be dissolved fromthe aluminium particles in the reaction before the solution is saturatedand the fluoride complex begins to form on the surface of the particles.If the treatment solution contains a complex fluoride even from thebeginning, the need for dissolved aluminium decreases and theprecipitation of the fluoride complex starts more quickly.Hexafluorosilicate in the solution is incorporated in the formedfluoride complex together with in situ formed and optionally addedhexafluoroaluminate. The presence of complex fluoride in the treatmentsolution results in a denser coating, which makes the aluminium morestable to, for instance, moisture and air. The surface layer can also bemade thicker, which is particularly important when relatively coarsealuminium particles (>80 μm) are treated. A higher treatment temperaturecan be used when complex fluoride is present in the treatment solution.The solubility of complex fluorides increases with temperature and at+40° C. or higher, it is so high that it can be difficult to obtain astable coating if the solution contains only hydrogen fluoride andalkali metal fluoride. The aluminium particles will be highly corrodedinstead. If the treatment solution is saturated in advance with complexfluoride, this problem disappears. The treatment temperature can beraised and thicker layers of fluoride complex can be produced.

If use is made of alkaline earth metals as counter-ion to the fluorideor the complex fluoride in the treatment solution, a surface layer willbe obtained, which is much less soluble than in the case when alkalimetal is used as counter-ion. This can be utilised in many ways. If, forinstance, a very fine powder is available, and only as thin a coating offluoride complex as possible is required, it is convenient to use analkaline earth metal, such as magnesium, calcium or strontium, ascounter-ion. Alkaline earth metal complex fluorides, for instancemagnesium hexafluoroaluminate or magnesium hexafluorosilicate, can beadded to the treatment solution when producing extremely thin surfacelayers. Extremely thin layers can be produced by low concentrationsbeing used in the treatment solution. The presence of alkaline earthmetal complex fluoride causes all dissolved aluminium to immediatelyprecipitate as fluoride complex on the particles in spite of the lowconcentration.

The treatment solution may contain, in addition to hydrogen fluoride,mixtures in all combinations of two or more of alkali metal fluoride,alkaline earth metal fluoride, alkali metal complex fluoride andalkaline earth metal complex fluoride. Different combinations can beused to obtain a desired layer structure and layer thickness.

The layer structure can be controlled by the process being performed inseveral steps by components, which result in a fluoride complex which ismore difficult to dissolve, being added to the treatment solution as thetreatment proceeds. The process can be started, for instance, with atreatment solution containing HF and alkali metal fluoride, after whichalkaline earth metal fluoride and/or alkaline earth metal complexfluoride is added. Alkaline earth metal ions can also be added in thefinal stage of the process to “empty” the solution of dissolved complexfluoride, which causes a thicker surface layer on the aluminiumparticles and a smaller amount of remaining fluoride in the wastesolution. The added alkaline earth metal salt does not, in that case,have to be a fluoride or complex fluoride but can be an arbitrary,soluble alkaline earth metal salt, such as calcium chloride.

TG analyses performed in oxygen demonstrate that after treatment thefuel particles are at least three, and in some cases fifty, times morereactive than before treatment. Aluminium powder treated in this mannercan be ignited in air by being heated with a Bunsen burner, which is notpossible with untreated aluminium.

The invention will be described below by way of examples and TG graphs(Thermogravimetric) for the aluminium fuel particles produced accordingto the Examples and, for comparison, TG graphs for untreated aluminiumpowder and aluminium powder treated with hydrofluoric acid only. The TGgraphs illustrate the percentage increase of the weight of the particlesas a function of the temperature when heating a particle sample inoxygen atmosphere at a selected constant heating rate. The increase inweight indicates how the sample has oxidised at different temperatures.188% corresponds to a complete oxidation of the aluminium powder toAl₂O₃. In all cases, the heating rate was 20° C./min; the initialtemperature was 100° C. and the final temperature 1100° C. The supply ofoxygen was 50 ml/min in all cases.

FIG. 1 shows a TG graph for untreated Al powder;

FIG. 2 shows a TG graph for Al powder treated with hydrofluoric acid;

FIG. 3 shows a TG graph for Al powder with a surface layer of trisodiumhexafluoroaluminate (cryolite) according to the invention;

FIG. 4 shows a TG graph for Al powder with a surface layer oftripotassium hexafluoroaluminate according to the invention;

FIG. 5 shows a TG graph for Al powder with a surface layer oftrirubidium hexafluoroaluminate according to the invention;

FIG. 6 shows a TG graph for Al powder with a surface layer of tricesiumhexafluoroaluminate according to the invention.

EXAMPLE 1

A solution of HF and NaF at a molar ratio of 1:1 was added to asuspension of aluminium powder (Carlfors Bruk A100) in pure water at atemperature of 30° C. The total concentration of fluoride in thesolution was 0.5 M and the total amount of added fluoride was 2% of themolar amount of the aluminium. The fluoride solution was added to thesuspension of aluminium powder under rigorous agitation. The rate ofadding was controlled to prevent excessive frothing owing to generationof hydrogen. As soon as all the fluoride solution had been added,agitation was stopped and the powder was filtered off and dried. Asurface layer of trisodium hexafluoroaluminate (cryolite) had formed onthe particles. FIG. 3 shows the TG graph for the thus treated powder. Asis evident from the graph, reactivity increases at a lower temperaturecompared with untreated powder (FIG. 1) and powder treated with HF only(FIG. 2).

The method was repeated with suspensions of aluminium powder in purewater at temperatures between 0° C. and 100° C. Owing to the solubilityof the formed fluoride complex at a higher temperature, a treatmenttemperature of from 25° C. to 40° C. was preferred. The totalconcentration of fluoride in the added solution varied between 0.01 and10 M. A preferred concentration was 0.1-5 M and preferably 0.2-1 M. Thetotal amount of added fluoride varied between 0.01% and 10% of the molaramount of the aluminium. A preferred addition was 0.1-5% and mostpreferably 0.5-2%.

Powder treated in this manner burnt 5-10 times more quickly thanuntreated powder of the same particle size and shape.

EXAMPLE 2

Repeated treatments were made by a solution of HF and KF at a molarratio of 1:1 being added to suspensions of aluminium powder (CarlforsBruk A100) in pure water at temperatures from 10° C. to 80° C. The totalconcentration of fluoride in the treatment solutions varied between 0.1and 5 M and the total amount of added fluoride was 0.1-5% of the molaramount of the aluminium. The fluoride solution was added to thesuspension of aluminium powder under vigorous agitation, and the powderwas filtered off and dried as soon as all fluoride solution had beenadded. A surface layer of tripotassium hexafluoroaluminate formed on theparticles. FIG. 4 shows the TG graph for the thus treated powder wherethe total amount of added fluoride was 2%.

Powder treated in this manner burnt 10-20 times more quickly thanuntreated powder of the same particle size and shape.

EXAMPLE 3

Example 2 was repeated, but with a treatment solution consisting of HFand LiF. FIG. 5 shows a TG graph for a thus treated powder where thetotal amount of added fluoride was 2%. The powder burnt 10-20 times morequickly than untreated powder of the same particle size and shape.

EXAMPLE 4

Example 2 was repeated, but with a treatment solution consisting of HFand RbF. FIG. 6 shows a TG graph for a thus treated powder where thetotal amount of added fluoride was 2%. The powder burnt 10-20 times morequickly than untreated powder of the same particle size and shape.

EXAMPLE 5

Example 2 was repeated, but with a treatment solution consisting of HFand CsF. FIG. 7 shows a TG graph for a thus treated powder where thetotal amount of added fluoride was 2%. The powder burnt 10-20 times morequickly than untreated powder of the same particle size and shape.

EXAMPLE 6

A solution containing HF and KF (at a molar ratio of 1:1) and H₂SiF₆ wasadded to a suspension of aluminium powder (Carlfors Bruk A100) in purewater at a temperature of about 80° C. The fluoride/complex fluoridesolution was added to the suspension of aluminium powder under vigorousagitation. As soon as all the treatment solution had been added,agitation was stopped and the powder was filtered off and dried. Afluoride complex containing hexafluoroaluminate and hexafluorosilicatehad formed on the particles. The procedure was repeated at differenttemperatures between 0° C. and 100° C. of the aluminium powdersuspension. A higher treatment temperature resulted in a thicker surfacelayer.

Powder treated in this way burnt 30-50 times more quickly than untreatedpowder of the same particle size and shape.

EXAMPLE 7

Example 6 was repeated, but instead of H₂SiF₆, SiO₂ in fine powder formwas added to the treatment solution. The silicon dioxide reacted withthe other components in the treatment solution and formedhexafluorosilicate in situ. A fluoride complex of the same type as inExample 6 formed on the particles.

EXAMPLE 8

Example 2 was repeated, but a CaCl₂ solution was added at the end of thetreatment. A thicker surface layer formed when CaCl₂ was added to thetreatment solution than in the case where merely HF and KF were used atthe same temperature. The fluoride complex on the particles containedpotassium and calcium hexafluoroaluminate. Powder treated in this mannerburnt 30-50 times more quickly than untreated powder of the sameparticle size and shape. The residual solution from the treatmentcontained very small contents of dissolved hexafluoroaluminate.

EXAMPLE 9

A solution containing HF and KF (at a molar ratio of 1:1), which hadbeen saturated with Na₃AlF₆, was added to a suspension of aluminiumpowder (Carlfors Bruk A100) in pure water at a temperature of about 80°C. The fluoride/complex fluoride solution was added to the suspension ofaluminium powder under vigorous agitation. As soon as all the treatmentsolution had been added, agitation was stopped and the powder wasfiltered off and dried. The procedure was repeated at differenttemperatures between 0° C. and 100° C. Thicker layers of trisodiumhexafluoroaluminate (cryolite) were obtained when the treatment solutionhad been saturated in advance with cryolite, and thicker layers could beprovided at higher temperatures.

Powder treated in this manner burnt 10-20 times more quickly thanuntreated powder of the same particle size and shape.

1. A method of producing aluminum fuel particles having improvedignitability and burn rate, comprising treating the aluminum fuelparticles with an aqueous solution of hydrofluoric acid and (i) afluoride or (ii) a complex fluoride of at least one of an alkali metaland an alkaline earth metal to form a surface layer of a fluoridecomplex bound to the aluminum fuel particles.
 2. The method as claimedin claim 1, wherein an alkaline earth metal ion is added to the aqueoussolution in a final stage of the treatment.
 3. The method as claimed inclaim 1, wherein the alkali metal fluoride is selected from the groupconsisting of sodium, potassium, rubidium, and cesium fluoride.
 4. Themethod as claimed in claim 1, wherein the complex fluoride is ahexafluoroaluminate or a hexafluorosilicate.
 5. The method as claimed inclaim 1, wherein the alkali metal fluoride is sodium fluoride and thefluoride complex is cryolite.
 6. The method as claimed in claim 1,wherein the alkali metal fluoride is potassium fluoride and the fluoridecomplex is tripotassium hexafluoroaluminate.
 7. Aluminum fuel particlesfor propellant and explosive compositions and pyrotechnic charges, saidaluminum fuel particles comprising a surface layer of a fluoride complexprovided by treatment of the aluminum fuel particles with an aqueoussolution of hydrofluoric acid and (i) a fluoride or (ii) a complexfluoride of at least one of an alkali metal and an alkaline earth metal.8. The aluminum fuel particles as claimed in claim 7, wherein the alkalimetal fluoride is selected from the group consisting of sodium,potassium, rubidium, and cesium fluoride.
 9. The aluminum fuel particlesas claimed in claim 7, wherein the complex fluoride is ahexafluoroaluminate or a hexafluorosilicate.
 10. The aluminum fuelparticles as claimed in claim 7, wherein the alkali metal fluoride issodium fluoride and the fluoride complex is cryolite.
 11. The aluminumfuel particles as claimed in claim 7, wherein the alkali metal fluorideis potassium fluoride and the fluoride complex is tripotassiumhexafluoroaluminate.